Carbon nanotubes (CNTs) are being investigated by a number of companies and institutions because of their unbelievable physical, chemical, electronical, and mechanical properties (Walt A. de Heer, “Nanotubes and the Pursuit of Applications,” MRS Bulletin 29(4), pp. 281-285 (2004)). They can be used as excellent cold electron sources for many applications such as displays, microwave sources, x-ray tubes, etc., because of their excellent field emission properties and chemical inertness for very stable and low voltage operation with long lifetime (Zvi Yaniv, “The status of the carbon electron emitting films for display and microelectronic applications,” The International Display Manufacturing Conference, Jan. 29-31, 2002, Seoul, Korea). Aligned carbon nanotubes have been demonstrated to have excellent field emission properties, which can be made by chemical vapor deposition (CVD) on a catalyst-supported substrate at over 500° C. (Z. F. Ren, Z. P. Huang, J. W. Xu et al., “Synthesis of large arrays of well-aligned carbon nanotubes on glass,” Science 282, pp. 1105-1107 (1998)). But the CVD process is not a good way to grow CNTs over large areas because it is very difficult to achieve high uniformity required for display applications. CVD growth of CNTs also requires a high process temperature (over 500° C.), eliminating the use of low-cost substrates such as soda-lime glass.
An easier process is to collect the CNT powders and uniformly deposit them onto selective area of the substrates. CNTs can be printed through a mesh screen if they are mixed with a binder, and epoxy, etc. (D. S. Chung, W. B. Choi, J. H. Kang et al., “Field emission from 4.5 in. single-walled and multiwalled carbon nanotube films,” J. Vac. Sci. Technol. B18(2), pp. 1054-1058 (2000)). CNT's can be sprayed onto the substrates if mixed with a solvent such as IPA, acetone, or water (D. S. Mao, R. L. Fink, G. Monty et al., “New CNT composites for FEDs that do not require activation,” Proceedings of the Ninth International Display Workshops, Hiroshima, Japan, p. 1415, Dec. 4-6, 2002). Special surface treatments are then often needed to achieve low electric field emission and high emission site density of the CNT cathodes. Hydrogen plasma etching (Jihua Zhang, Tao Feng, Weidong Yu et al., “Enhancement of field emission from hydrogen plasma processed carbon nanotubes,” Diamond and Related Materials 13, pp. 54-59 (2004)), ultraviolet laser irradiation (W. J. Zhao, N. Kawakami, A. Sawada et al., J. Vac. Sci. Technol. B21(4), pp. 1734-1736 (2003)), Magnesium oxide thin-film deposition at the top of the CNT layer (Won Seok Kim, Whikun Yi, SeGi Yu, et al., “Secondary electron emission from magnesium oxide on multiwalled carbon nanotubes,” Appl. Phys. Lett. 81(6), pp. 1098-2000 (2002)) are effective ways to improve field emission from of the CNTs. But none of them can be processed on large areas uniformly. A taping process seems to be an attractive way to enhance the field emission properties of the carbon nanotubes (Yu-Yang Chang, Jyh-Rong Sheu, Cheng-Chung Lee, “Method of improving field emission efficiency for fabricating carbon nanotube field emitters,” U.S. Pat. No. 6,436,221). In this method, an adhesive tape is closely attached on the CNT cathode substrate and then it is removed. Some carbon nanotubes will be vertically oriented, and those poorly bonded CNT portions will be removed. It is highly possible that some adhesive residue will remain on the substrate and the top of the carbon nanotube layer. The organic residue on the substrate after the taping activation process may give off undesirable residual gases in the sealed glass display envelope during field emission operation. Furthermore, it is difficult to uniformly activate the substrate over large areas. For example, many display applications may require 40-100 inch diagonal plates. All of these problems obviously obstruct the various field emission applications of CNTs.